Laminate insulation



Oct. 25, 1932. s g s 1,884,311

LAMINATE INSULATlON Filed Jan. 9, 1929 mama 0a. 25, 1932 UNITED STATESPATENT OFFICE DONALD M. SIMMONS, OF OSBORNE, PENNSYLVANIA, ASSIGNOR TOGENERAL CABLE CORPORATION, OF NEW YORK, N. Y., A CORPORATION OF NEWEBSEY LAMINATE INSULATION Application filed January 9, 1929. Serial No;331,172.

My invention relates to improvements in insulation for electricconductors, and particularly to insulation which includes laminae ofpaper, cloth, or other porous material, filled with insulatingsubstance. The insulating substance when applied is in liquld condition.A typical case is cable insulation formed of paper tape wrapped to placeand filled with insulating compound. The object of my improvements is tostrengthen such a body of insulation against breakdown.

The accompanying drawing is a fragmentary view of a cable, showing inelevatlon the conductor itself, and diagrammatically and in axialsection a body of surrounding insulation built in embodiment of myinvention. The paper or other porous material which forms the immobileportion of such an insulating envelope as I have indicated is ordinarilyapplied in the form of a ribbon or tape of relatively great width andsmall thickness, and in the building up of the laminze one upon anotherspaces of relatively small width are commonly formed between thesuccessive turns of tape in any given lay er; for, ordinarily, in thelaying of the tape the adjacent edges of successive turns are spacedapart at a slight interval. In the building of cables particularly, thetape is laid spirally around the conductor, and the spacing apart of theedges of successive turns 1s a feature of value and importance, since itafiords in the finished cable superior flexibility. The cable may bebent without undesired wrinkling of the paper of which the body ofinsulation is fundamentally composed. ln the building of layer uponlayer the spaces between succeeding turns in an underlying layer arecovered by the continuous web of the next overlying layer. That is tosay, thesuccessive layers of paper or its equivalent are so superposedthat the spaces in one layer are offset with respect to those of anadjacent layer.

This is illustrated in the drawing. The cable conductor is indicatedat 1. Upon it the tape 2 is spirally Wrapped, and between the turns ofthe spiral spaces (a, c, a l, e, are left. The spaces 2') of the secondlayer are offset with respect to spaces a of the first layer; and thespaces 0 of the third layer are offset with respect to the spaces 6 ofthe second; and so on.

These spaces, indeed, are of small size. Even so, under practicalconditions of cable building, they are of sufficient size to present aproblem and a difliculty-which by my invention is solved and overcome.

The wrapped-on body of paper or the like, after application, is filledwith insulating material in fluid condition. All the pores are occupied,and the spaces a, b, 0, etc. as well.

Generally speaking, insulation so built is the best that the art knowsfor the purpose.

In the transmission of power at high voltage every weakness is searchedout, and it becomes a matter of engineering to design a cable of minimumdimensions and cost to carry, through long periods of time, current ofgiven maximum power.

s between a body of paper or equivalent fibrous material filled with oilor other fluid or viscid insulation and a free body of such fluid orviscid insulation of like dimensions, the former is, dielectricallyconsidered, much the stronger. And, with attention confined to freebodies of liquid or viscid insulation, the unit dielectric strength ismuch greater, relatively speaking, in a thin layer than in a layer ofgreater thickness. In structures such as that under consideration, allbodies of compound which fill spaces otherwise void are weaker than thebodies of compoundfilled paper, and the bodies of compound which fillthe spaces a, b, 0, etc.. since they are of much greater radial depththan any other bodies of free compound within the structure, are by farthe weakest portions of the whole insulating envelope.

As cables such as that under consideration continue in use, there is aptto be diminution in compound-whether because of i and more completeabsorption by the rous bodies, or in consequence of flow tc otherregionsand consequent voids are apt to appear. voids, wherever theyoccur, are, beyond ali else, weaker.

lie the transmission of alternating the stress upon the insulation isdist,

in inverse proportion to relative specific inductive capacity. That isto say, at points where the specific inductive capacity of theinsulating wall is less, the strain tending to break-down is greater.

Coming to conditions of actual practice, the body of compound-filledporous paper may have a specific inductive capacity of about 3.5;whereas free bodies of liquid or viscid insulation if present within thespaces a, b, 0, etc. may have a specific inductive capaclty of 2 orslightly more than 2; and voids, if voids are found, have a specificinductive capacity of 1.

Therefore the tendency to break-down throu h the oil-filled s aces a, b,0, etc. (if, indee ,these spaces be lled with oil), which exists becauseof the inferior strength of free bodies of oil to resist break-down, ismtensified, because of the concentration of stress upon these regions oflow specific inductive capacity. And in the use of cables whoseinsulation is of the character described, whether the current carried bedirect or alternating, it is found that the cable tends to break downrecisely at the compoundfilled spaces. It is there that, under excessivestress, ionization occurs and charring begins, accompanied sometimes andperhaps facilitated by chemical change in the insulating compounditself.

My invention consists in a modification of structure, whereby break-downstrength at the weakest points is increased and the efl'ect ofdifferences in specific inductive capacity of materials is diminished.

In reference made above to the drawing, I have for purposes ofdiscussion, assumed that in the completed cable the spaces a, b, 0, etc.were filled with insulating compound alone. Such is not, in fact, thecase. In the wrapping of the paper tape I wrap at the same time a strandof fibrous material, placing it between the spaced-apart edges of thetape, so that in the completely wrapped structure it occupies the spacesa, I), 0, etc. The advantages of the spaces left in the course ofwrapping between the edges of successive turns of the tape still remain.When the body of insulation is completed by impregnation with insulatingcompound of the immobile portion of the envelope, the bodies of compoundwhich otherwise would wholly occupy the spaces (1,7), 0, etc. are nolonger free bodies. Their continuity as free bodies of compound isinterrupted by the fibres of the strands there introduced. By suchprovision the breakdown strength of the structure as a whole issubstantially increased.

The specific inductive capacity of such introduced strand of fibrousmaterial is in any.

case greater than that of the insulating compound (and much greater thanthat of any void which might at that point develop), and accordingly,the concentration of dielectric stress is by the presence of theintroduced strands diminished.

The introduced strand may be a twine of hemp or other vegetable fibre,impregnated with insulating compound; The specific inductive capacity ofsuch twine is approximately that of paper; and, since the twine by itspresence will effect the breaking up of the continuity'of the bodies ofcompound within the spaces, the electrical strength of the structurewill be greatly increased. In

any case it'will be perceived that, whereas the wrapped on strips ofpaper which the art knows are practically incompressible longitudinally,the strand applied in the manner described and consisting of materialsuch as I have indicated is compressible, to the end that, whileachieving the result which makes this invention useful, it still allowsthe cable to be flexed.

The strength of the-structure as a whole may be increased still more bythe selection of the material for the introduced strand. If, forexample, twine of hem or other vegetable fibre be used, impregnate witha phenol condensation product, whose specific inductive capacity ishigh, the tendency to localized concentration of stress will be inlarger measure corrected; or again, if the strand be a strand of metalwool (the sepcific inductive capacity of metal being infinite in value)the limiting case is found so far as concerns range of availablematerials) and the tendency to concentration of stress will besubstantially eliminated. A structure which includes metal wool sointroduced is specifically claimed in a companion application filed byme on even date herewith, Serial No. 331,171, upon which applicationLetters Patent No. 1,775,072 were issued September 2, 1930.

And, as has already been said and repeated, the breaking up of thecontinuity of the bodies of compound by fibres of solid material in anycase increases the strength of the structure to resist break-down.

I claim as my invention:

1. In an electrical installation each layer of which is laminateinsulation formed of strips of fibrous material of relativel great widthand small thickness laid side y side with spaces of relatively smallwidth between the edges of adjacent strips, the spaces in one layerbeing ofiset relatively to the spaces in an adjacent layer, and strandsof fibrous material laid in the spaces between adjacent strips, thewhole being filled with insulating compound.

2. In an electrical installation laminate insulation formed of strips offibrous material laid side by side with spaces between the edges ofadjacent strips, strands of fibrous material impregnated with a materialof a high s cific inductive capacity laid in the spaces etween adjacentstrips, the whole being filled with material of lower specific inductivecapacity than said first material.

3. In an electric cable structure a conductor, a body of wrapped-onporous tape of relatively great width and small thickness envelopingsaid conductor, the turns of ta pc spaced apart edge from edge by spacesof relatively small width a strand of fibrous material laid in the spacebetween adjacent turns of tape, the whole being filled with insulatingcompound.

4. An electric cable structure including a conductor and an envelope ofinsulation surrounding the conductor, said envelope consisting of a.wrapped-on body of oil-filled porous material, adjacent turns of theWrapped-0n body being spaced apart and the intervals between such spacedapart turns being occupied with insulation of specific inductivecapacity greater than that of the remaining portions of the envelope.

5. In an electric cable structure a conductor, a body of insulationcomprising overlying layers of spirally wrapped strip material, theedges of adjacent turns in said layers being separated by relativelynarrow spaces, and strands of fibrous material laid in the spacesbetween adjacent turns of a layer.

6. In an electrical installation laminate insulation formed of strips offibrous material relatively incompressible in the direction of thelength of the structure laid side by side with spaces between the edgesof adjacent strips, and compressible strands of fibrous material laid inthe spaces between adjacent strips, the whole being filled withinsulating compound.

In testimony whereof I have hereunto set my hand.

DONALD M. SIMMONS.

